Method of operating an ion microprobe using secondary elections



Nov. 18. 1969 H. J. Ll L METHOD OF OPERATING ION MICROPROBE LECTIONSUSING SECONDARY E Filed June 30, 1966 TZI FIG.2 x 6 Ill-1h- INVENTOR.HELMUT J. LIEBL ATTORNEY United States Patent 3,479,505 METHOD OFOPERATING AN ION MICROPROBE USING SECONDARY ELECTIONS Helmut J. Liebl,Goleta, Califi, assignor to Applied Research Laboratories, Inc.,Glendale, Calif., a corporation of Delaware Filed June 30, 1966, Ser.No. 561,997 Int. Cl. H0lj 37/26 U.S. Cl. 250-495 3 Claims ABSTRACT OFTHE DISCLOSURE This invention relates to a novel method of operating anion probe of the general type described and claimed in my copendingapplication for patent, Ser. No. 494,388, filed Oct. 11, 1965, andentitled, Ion Microprobe, and more particularly, to a novel method ofoperation which facilitates the precise determination of the spot on thesurface of a specimen where the ion beam impinges.

In general, an ion probe of the type with which the present invention isconcerned includes an ion source, means for accelerating ions away fromthe source toward a specimen to be analyzed, means for focusing the ionsso accelerated into a relatively narrow beam so that they impinge onlyupon a very small area of the specimen, and usually an ion analyzer foranalyzing ions sputtered from the surface of the specimen by the ionbeam. Instruments of this type are capable of great sensitivity in thechemical analysis of materials. They have been shown to be able todetect elements that are present in a specimen material in amounts ofonly a few parts per billion.

One problem in the operation of an ion microprobe of the stated type isto determine with precision the exact spot where the ion beam impingeson the specimen at any given moment. It is not a ditficult matter toarrange a microscope in the apparatus for visual observation of thespecimen while it is under bombardment by the ion beam, but by itselfthis does no good, because the ion beam produces no immediate, visibleindication of its presence. There is no glow at the point ofimpingement, nor any immediately apparent visible change in the natureof the surface.

In the operation of the ion probe as described in the hereinaboveidentified copending application, the ion beam may be scanned in araster pattern over a limited portion of the surface of a specimen,while a similar pattern is produced synchronously upon the face of anoscilloscope. An electrical signal is produced indicative of theinstantaneous ion emission at a selected mass and energy from thesurface of the specimen, and is applied in amplified form to modulatethe intensity of the electron beam of the oscilloscope. The oscilloscopethus produces a light image indicating the variations in concentrationof a particular element, which image can, in many cases, be matched withan image of the surface of the specimen as seen through a microscope.The matter becomes difficult, however, when operating the microprobewith very small currents in the ion beam, because the secondary ions,which are collected to produce the electrical signal, are then very few,and the image on the oscilloscope is apt to 3,479,505 Patented Nov. 18,1969 lack adequate detail for easy matching with the image in themicroscope.

Accordingly, an important object of the present invention is to improvethe operation of an ion microprobe of the stated type, and to facilitatethe determination of the point at which the ion beam imjinges upon aspecimen at any given moment.

Briefly, according to the invention, it has now been found that ifelectrons emitted by the specimen are collected and used to produce thesignal for modulating the oscilloscope, an image of improved clarity anddetail can be produced, thereby facilitating the determination of thelocation of the ion beam upon the surface of the specimen.

The number of electrons emitted by the specimen in response to ionbombardment greatly exceeds the number of ions emitted of any given massand energy. For a given beam current, therefore, a larger electricalsignal can be obtained from the electron emission than from the emissionof only selected ions. This is particularly true in those cases wherethe selected ions are those of elements that are present in the specimenonly in very small proportions. The electron emission is also a functionof the composition of the surface, and has been found to vary with fullyadequate sensitivity to enable ready matching of the oscilloscope imagewith the image in the microscope.

The practice of the invention is expected to find its greatestutilization in connection with use of the ion probe for work other thanion analysis such as, for example, when the probe is used for etching amaterial by sputtering, or for depositing a layer of a material from theion beam upon a selected area of a body, or when it is used forinspecting bodies such as miniature semiconductor devices for defects.

Different materials in or on the surface of the specimen emit electronsat different respective rates in response to ion bombardment of a givenmagnitude. Compositional irregularities such as grain boundaries inmaterials, for example, emit electrons at different rates from otherportions of the specimen, and thus show up in the electrically producedimage. Many such irregularities are also visible when the surface isviewed through a microscope, so that an observer is enabled to locatevery precisely the position of the ion beam at any given moment. In theevent that naturally occurring irregularities are insufficient to enableready identification of the desired area on the specimen, a selectedmaterial different from the major constituent materials of the specimenmay be applied to the surface of the specimen in accordance with apredetermined pattern to provide positively identifiable referencepoints or areas.

A presently preferred embodiment of the invention will now be describedin connection with the drawing, Where- FIGURE 1 is a fragmentary,schematic diagram of a portion of an ion microprobe arranged for thepractice of the presently preferred embodiment of the invention, and,

FIGURE 2 is a similar view to that of FIGURE 1, but showing themicroprobe arranged for the practice of an alternative form of theinvention.

The drawing shows only that portion of the microprobe which must bemodified for the practice of the present invention from the arrangementshown in my hereinabove identified copending application, and thatapplication may be referred to for a complete description of anoperative ion microprobe of the kind with which the present invention isprimarily concerned. As shown in FIGURE 1, a beam 10 of ions generatedby any desired means is directed toward a specimen material 12 andimpinges thereon in a spot of microscopic dimensions. The beam 10 passesbetween two pairs of deflection plates 14 and 16, respectively, and isthen focused by a uni-potential lens 18 upon the surface of the specimen12. Means (not shown) are provided for applying deflection voltages tothe plates 14 and 16 to cause the beam 16 to scan across a pre-selectedarea of the surface of the specimen. Such scanning may be in the mannerof a television raster, or, alternatively, under manual control, as in amanually driven cross-slide.

Ordinarily, as heretofore operated for analysis of secondary ionemission, the outer plates 20 and 21 of the uni-potential lens, whichserves as the objective lens for focusing the ion beam, are grounded,and the specimen is biased positively about 2 /2 kv. with respect toground. Positive ions sputtered from the surface of the specimen 12 arecollected by a grounded tubular electrode 28, and directed into a doublefocusing mass spectrometer for analysis.

In the practice of the invention in its preferred form,

the specimen is biased negatively with respect to ground by a fewhundred volts, and the electrode 21 of the objective lens 18 nearest thespecimen 12 is ungrounded and connected to the input of a DC amplifier24. Thus, electrons emitted by the specimen are accelerated toward andcollected by the adjacent electrode 21 of the objective lens. Thecurrent thus produced constitutes an electrical signal representative ofthe instantaneous rate of electron emission.

The signal is amplified by the amplifier 24 and applied to the controlgrid of an oscilloscope 26 for modulating the intensity of its trace.The electron beam of the oscilloscope 26 is detected synchronously withthe deflection of the ion beam 10, but, of course, across a much widerrange so that the raster on the oscilloscope, when raster scanning isused, represents a greatly enlarged image of the scanned portion of thesurface of the specimen. The image produced by the oscilloscope is thencompared with the optical image produced by a microscope (not shown)aimed at the specimen, thus enabling precise determination of theposition of the ion beam at any given moment.

Image's produced by the oscilloscope in the practice of the inventionare sharp and clear even with currents of 10- ampere in the ion probe.It is also possible, by using methods of accelerating the electrons fromthe specimen, and by having them impinge on a scintillation crystal, theoutput of which is fed to a photomultiplier, to obtain useful images onthe oscilloscope when the current in the ion beam is even smaller than10" ampere. One such arrangement for accelerating the electrons andusing a scintillation crystal is described in an article by T, E.

Everhart and R. F. M. Thornley entitled Wideband Detector forMicro-Microampere Low-Energy Electron Currents in the Journal ofScientific Instruments 37, pp. 246-248 (1960).

In accordance with the second embodiment of the invention as shown inFIGURE 2, the secondary electrons from the specimen 12 are collected bythe electrode 28 that is normally used for accelerating positive ionsfrom the specimen into mass spectrometer 30. In this case, the frontelement 21 of the objective lens is left grounded, and the collectingelectrode 28 is ungrounded and connected to the input of the amplifier24. The specimen again is biased a few hundred volts negatively.Substantially the same results are achieved in this as in the firstembodiment of the invention. The selection of an electrode forcollecting the electrons is largely a matter of convenience.

In both of the herein described embodiments of the invention, it ispossible to change the bias voltage to suit the operators or designersconvenience, provided only that the collecting electrode is maintainedpositive with respect to the specimen. It is believed that thearrangements described herein, however, are probably the most convenientand simple in the present state of the art.

What is claimed is:

1. Method of operating an ion probe of the type wherein a beam of ionsis focussed upon a microscopically small area of a specimen and scannedacross the specimen to bombard successive incremental portions thereofto facilitate determination of the points of impingement of the beam onthe specimen, said method comprising the steps of collecting electronsemitted by the specimen in response to bombardment by the ion beam,producing an electrical signal indicative of variations in the numbersof electrons emitted from the successive incremental portions as thebeam is scanned, and producing a visual display in response to theelectrical signal so produced, thereby enabling a comparison between thevisual display and an optically produced image of a portion of thespecimen.

2. Method of determining the point of impingement of an ion beam uponthe surface of a specimen comprising the steps of scanning a sharplyfocussed ion beam across a selected portion of the specimen, producingan electrical signal responsively to electrons emitted by the surface inresponse to the ion beam, the electrical signal being indicative of theinstantaneous rate of electron emission from the specimen, and producinga visual display in response to the electrical signal so produced,thereby enabling a comparison between the visual display and anoptically produced image of the selected portion of the specimen.

3. Method in accordance with claim 2 wherein the electrons are collectedby an electrode arranged symmetrically around the ion beam wherebychanges in the potential of the electrode do not substantially divertthe ion beam.

References Cited UNITED STATES PATENTS 3,103,584 9/1963 Shapiro et al.25049.5 3,219,817 11/1965 Mollenstedt 25049.5

OTHER REFERENCES Mass-Spectrometric Micro-Surface Analysis by GeophysicsCorporation of America, Bedford, Mass., received Mar. 25, 1963, 5 pp.

Focused Slow Ion Beam for Study of Electron Ejection from Solids by H.D. Hagstrorn et al., from Review of Scientific Instruments, vol. 36, No.8, August 1965, pp. 1183-1190.

WILLIAM F. LINDQUIST, Primary Examiner

